The present invention relates to an ultrasound diagnostic apparatus and a method for processing data of ultrasonic echoes, particularly to an ultrasound diagnostic apparatus and data processing method capable of obtaining high-quality image regardless of the difference in sound velocity in a subject's body.
Ultrasound diagnostic apparatuses using ultrasound images are put to practical use in the medical field.
Generally, an ultrasound diagnostic apparatus of this type comprises an ultrasound probe (hereinafter called “probe”) and a diagnostic apparatus body. In such an ultrasound diagnostic apparatus, a probe transmits an ultrasonic wave toward a subject's body, the ultrasonic echo from the subject is received by the probe, and the resulting reception signals are electrically processed by a diagnostic apparatus body to generate an ultrasound image.
In conventional ultrasound diagnostic apparatuses, the sound velocity of ultrasonic waves is assumed to be constant in a living body of a subject, and a sound velocity value of ultrasonic waves is fixed to a certain value.
However, the sound velocity varies depending on the type of tissues of a fatty layer, a muscular layer and the like, and therefore the sound velocity of ultrasonic waves is not constant in a subject. Further, fat people and slim people are different in thickness of a fatty layer and a muscular layer. In other words, the sound velocity varies from person to person.
Accordingly, in the case of a conventional ultrasound diagnostic apparatus in which the sound velocity of ultrasonic waves is fixed, when the actual sound velocity in a subject differs from a set sound velocity, the arrival time of an ultrasonic echo reflected inside the subject does not match with a set delay time.
This hinders appropriate phase matching, and reception focusing is not properly performed, so that the image quality of the resulting ultrasound image deteriorates. In addition, there is a problem in that the resulting ultrasound image is distorted relative to the actual subject.
To cope with such a problem, JP 2011-92686 A describes an ultrasound diagnostic apparatus that sets a region of interest, transmits and receives ultrasonic waves, calculates a focus index, which is brightness for instance, for each of plural, appropriately set sound velocities by performing reception focusing on an acquired reception signal with the use of the sound velocities, and determines a sound velocity at the region of interest using the calculated focus index, for instance, determines a set sound velocity with the highest brightness, which is the focus index, as the sound velocity at the region of interest.
In the ultrasound diagnostic apparatus described in JP 2011-92686 A, as an example, the delay time and delay pattern is adjusted based on the determined sound velocity before the reception focusing is performed and an ultrasound image is generated. Thus, the ultrasound diagnostic apparatus is capable of steadily outputting an appropriate ultrasound image regardless of the difference in sound velocity inside a subject or among subjects.
JP 2009-100997 A describes an ultrasound diagnostic apparatus that uses the facts that the intensity of a reception signal (echo intensity) reflects the hardness of tissue at a reflection point in a subject and that a sonic speed varies depending on the hardness of tissue, thereby preventing the image quality from deteriorating due to the difference in sonic speed inside a subject.
When this ultrasound diagnostic apparatus is used, on the basis of the facts that the intensity of a reception signal can be an index of hardness of tissue and that the hardness of tissue and the sonic speed are correlated with each other to some extent, a ratio table indicative of the relation between the intensity of a reception signal and a sonic speed ratio, which is a variation of sonic speed relative to a set sonic speed, is prepared in advance. Thereafter, the sonic speed ratio is detected using the intensity of reception based on the ratio table, and pixel positions in an ultrasound image are rearranged using the sonic speed ratio to thereby perform sonic speed correction.